1. Field of the Invention
The present invention refers generally to the field of thermal energy storage, and more particularly to the storage of such energy by utilizing phase change materials. Phase change materials, hereinafter sometimes refer to as PCM's, are chemical compositions that store heat energy in one phase and, in another phase, either release such energy in the form of heat, or acquire heat from a surrounding fluid as the PCM undergoes a change of phase.
2. State of the Art
The field of thermal energy storage ("TES") is believed to be in its commercial infancy. While there are presently a plurality of installations in commercial operation in the United States, the full market potential of "coolness" storage, in particular, has yet to be achieved. In general, TES installations use PCM's to store coolness, rather than heat. Such PCM's are chemical compositions that exhibit a freezing point plateau at temperatures below about 50.degree. F. Since electric utilities almost universally employ time-of-use rates for industrial users, the rate being divided into a demand charge and a usage charge, where a commercial operation can employ electricity during off-peak hours rather than during peak periods, it is highly desirable to do so from an economic view. For off-peak usage, both the demand and energy charges that are levied for energy use during peak periods will be diminished and, instead, the same usage made during off-peak periods will result in a much lower total charge to the customer.
While water may be defined as a phase change material, it has long been apparent that one may freeze water during off-peak hours, such as the early morning hours when the load on the utility's generating capacity is minimal, then use the ice created during those off-peak hours, either directly or by melting and mixing with line water, during peak hours to air-condition an installation. At the present time there are many commercial entities seeking to capitalize on storage of "coolness" through the employment of a water-ice-water change of phase. However, the drawbacks of ice storage are manifest.
It will be readily apparent that energy usage is magnified when water is frozen, since in order to accomplish such freezing within a reasonable period of time, the freeze-inducing medium must be at a temperature well below that at which water freezes: a medium at a temperature of about 26.degree. F. must be employed, which means that the medium cannot be water, but usually is a mixture of water and ethylene glycol, i.e., antifreeze. The environmental hazards of such usage, as well as the fact that the energy employed to chill the cooling medium to 26.degree. F. is well in excess of that required to chill water to the desired temperature for air-conditioning, generally between 41.degree. and 47.degree. F., are apparent. Thus, ice storage does not result in conservation of energy, even though electric utilities can generally produce electricity more cheaply at night when ambient temperatures are low.
There has been widespread commercial use of one PCM other than water. A PCM that meets the general requirements for all PCMS: (1) maintenance of a near constant temperature throughout the phase change cycle, also referred to as a melting/freezing point plateau, (2) a relatively high latent heat of fusion, and (3) a relatively high product density; is based on a eutectoid salt composition employing sodium sulfate decahydrate. While such phase change materials employing Na.sub.2 SO.sub.4.10H.sub.2 O are readily available, such mixtures have a minimal freezing point of 47.degree.-48.degree. F. In many instances, it has been found desirable to employ water for air-conditioning at temperatures below 47.degree.-48.degree. F., for example, water at 41.degree. F. It will be apparent that, unless a chiller is to be used during peak periods to further cool water from a cooling tank employing sodium sulfate decahydrate-based PCM's, one cannot obtain water at 41.degree. F. when the melting point of the PCM is 47.degree. F., absent the inefficient utilization of the specific heat at a PCM frozen below its melting point.
As disclosed in the application of which this application is a continuation-in-part, it has more recently been learned that certain NaOH/H.sub.2 O solutions can be employed as a PCM for cool storage. Thus, an aqueous solution of about 46% -47% NaOH-in-water has a freezing/melting point plateau at approximately 41.degree. F., which in many instances is highly favorable vis-a-vis a sodium sulfate decahydrate PCM with a 47.degree. F. plateau. For uses where lower than 47.degree. F. water is desired for air-conditioning the NaOH/H.sub.2 O solution is exceptionally well-suited for employment as a PCM.
One problem that has been noted with the use of NaOH/H.sub.2 O PCM's is that they have significant tendency to supercool. In the prior, sole application a significant advance in the art was disclosed by providing several nucleating agents that inhibited such supercooling. Among those nucleating agents, and highly desirable because it is inexpensive and readily available, was magnetite, which is mineral, commercial grade of Fe.sub.3 O.sub.4, also sometimes known as magnetic iron oxide. While magnetite in all of its forms appears to be a step forward in its use as a nucleating agent for sodium hydroxide/water solutions and, in particular, for a 47% sodium hydroxide/water composition that exhibits a freezing point plateau at about 41.degree. F., simply placing the magnetite in the NaOH/water solution and lowering the temperature thereof below 41.degree. F. presents problems after many, repeated freeze/thaw cycles.
Although the use of finely divided magnetite powder is effective in nucleating an NaOH/water solution, some of that nucleating agent seems to supersaturate the PCM, itself, although under ambient circumstances, Fe.sub.3 O.sub.4 is only slightly soluble in dilute NaOH. In that case the magnetite had to be added to its saturation point, which appears to be about 3% by weight of the NaOH/water PCM. Then more magnetite had to be added so that a supersaturated solid always remained on the bottom of the container. Often, magnetite was added in about 4% to 6% by weight of the PCM. Since the magnetite nucleates, but does not take part in the freezing of the NaOH/water solution, and thus does not appear to contribute to the storage of coolness, the need to supersaturate the solution with magnetite is deemed undesirable.
Another problem that has been presented is that, as with any solid with a specific gravity greater than that of the PCM liquid, the magnetite would tend to sink to the bottom of the container for the PCM. When resting on the bottom, the magnetite was only presented to the lower portion of the PCM solution and, on occasion, tended to become "crusted over" after continued use. Whatever the material that formed on the surface of the magnetite, it inhibited the magnetite from performing its nucleating function.
Thus, one object of the present invention is to improve distribution of the magnetite or other nucleating agent throughout the body of the phase change material in which it is acting as a nucleating agent, thereby improving the efficacy of the nucleating agent.
It is another object of our invention to provide a nucleating agent that will be maintained in a PCM solution in a position other than precipitating to the floor of the PCM container, thereby presenting more faces on which nucleation can occur throughout the body of the PCM.
It is another object of our invention to provide a means for employing magnetite as a nucleating agent throughout a PCM in which the magnetite is maintained in a position in which it will not tend to crust over with other deposits, such has been found to occur when the magnetite remains on the bottom of the PCM container.
It is still another object of our invention to provide a means for positioning a nucleating agent in a body of PCM so that solubility of the nucleating agent in the PCM will be minimized and there will be no need to supersaturate the PCM solution with the nucleating agent, in this case magnetite.
In order to achieve the above objects, it has been determined that the magnetic properties of the magnetite can be utilized to suspend the magnetite or other ferromagnetic material in a more advantageous position where there will be far greater exposure of the particles of magnetite to the body of the NaOH/H.sub.2 O solution. This concept is embodied in various forms, and results in a device which greatly enhances the nucleating properties of the magnetite.